1. Field of the Invention
This invention pertains to methods and devices for the generation of very short pulses by lasers. More particularly, this invention pertains to the use of saturable absorbers for mode locking of lasers for the purpose of producing short pulses.
2. Description of the Prior Art
Short pulses which are produced by mode-locked lasers have many applications in biological studies, semiconductor studies, etc. In addition, if pulses could be produced of less than a picosecond in length and with an energy level in excess of 1 joule, it might be useful as a pump for a coherent, X-ray laser. (All X-ray sources to date are incoherent.) The generation of such pulses also would be useful for the holographic study of molecules.
Saturable dyes have been used extensively in the prior art to mode-lock lasers for the production of short pulses. Erich P. Ippen and Charles V. Shank have described the use of saturable dyes, for producing short pulses, in "Sub-Picosecond Spectroscopy", Physics Today, May, 1978, pp. 41-47, and have described the use of the dyes for mode-locking in "Mode-Locking of Dye Lasers" in Dye Lasers, edited by F. P. Schafer, Springer Verlag, New York, 1973.
In order to mode-lock lasers which contain a gain medium with a relatively short lifetime, a saturable absorber with a lifetime much shorter than the cavity round-trip time normally is required. Excellent results have been obtained when a dye like DODCI, which has a lifetime of a few hundred picoseconds, has been used to mode-lock RH6G (Rhodamine 6G), which has a lifetime of approximately 5 nanoseconds.
Unfortunately, in some wavelength ranges, and especially in the ultraviolet, saturable absorbers with such short lifetimes are not yet known, although absorbers with lifetimes on the order of several nanoseconds are easily available.
An object of this invention is to provide a mode-locking device which uses absorbers with lifetimes of several nanoseconds within a second resonator, whereby the "effective" lifetime of the absorber is reduced to the desired values of several hundred picoseconds.